Na + channel inactivation: a comparative study between pancreatic islet β‐cells and adrenal chromaffin cells in rat

A comparative study was carried out on the inactivation of Na + channels in two types of endocrine cells in rats, β‐cells and adrenal chromaffin cells (ACCs), using patch‐clamp techniques. The β‐cells were very sensitive to hyperpolarization; the Na + currents increased ninefold when the holding potential was shifted from −70 mV to −120 mV. ACCs were not sensitive to hyperpolarization. The half‐inactivation voltages were −90 mV (rat β‐cells) and −62 mV (ACCs). The time constant for recovery from inactivation at −70 mV was 10.5 times slower in β‐cells (60 ms) than in ACCs (5.7 ms). The rate of Na + ‐channel inactivation at physiological resting potential was more than three times slower in β‐cells than in ACCs. Na + influx through Na + channels had no effect on the secretory machinery in rat β‐cells. However, these ‘silent Na + channels’ could contribute to the generation of action potentials in some conditions, such as when the cell is hyperpolarized. It is concluded that the fractional availability of Na + channels in β‐cells at a holding potential of −70 mV is about 15 % of that in ACCs. This value in rat β‐cells is larger than that observed in mouse (0 %), but is smaller than those observed in human or dog (90 %).